1. Field of the Invention
This invention relates to ultrasonic transducers for use in ultrasonic diagnostic systems and more particularly, to the use of a specific type of polymer material for reinforcement of the transducer.
2. Description of the Prior Art
In the medical fields, ultrasonic diagnostic systems have been widely used in recent years. The ultrasonic diagnostic systems make use of a variety of ultrasonic transducers. Typical ultrasonic transducers are illustraed with reference to FIGS. 1(a) through 1(c) in which they are schematically shown.
Ultrasonic transducers shown in FIGS. 1(a) and 1(b) are of the single element type. In the figures, reference numerals 1, 2 indicate electrodes attached to a piezoelectric ceramic material 3 on opposite sides thereof, thereby giving a transducer element 4. The electrodes 1 and 2 have lead wires 5 and 6, respectively. On the electrode 2 is formed an acoustic impedance matcher 7 made of one or more layers. This matcher 7 serves to transmit an ultrasonic wave generated from the transducer element 4 in order to improve energy transfer between the high impedance piezoelectric ceramic material and the low impedance of human body being examined as is known in the art. The matcher 7 has an acoustic lens 8 on the side opposite to the electrode 2, by which the ultrasonic wave propagated through the acoustic impedance matcher 7 is focused and transmitted to the object being examined with an improved lateral resolution. In FIG. 1(a), a damping member 11 is provided in order to mechanically damp the transducer element 4 therewith.
FIG. 1(c) shows a linear transducer array. In this array, a multiplicity of transducer elements, e.g. several tens to several hundreds of elements, are linearly arranged on a plane.
The ultrasonic transducers having such constructions as described above are brought to contact with an object being examined at one surface of the acoustic lens 8 so as to transmit and receive ultrasonic waves, thereby diagnostically examining the object.
The acoustic impedance matcher 7 of the known ultrasonic transducers is usually constituted of one layer of a mixture of metal powder and a resin, or two layers including a first layer of glass and a second layer of plastic resin, with a thickness of as small as 0.2 to 0.5 mm. The acoustic lens 8 is made, for example, of silicone rubber and has a thickness as small as 0.5 to 1 mm. One of disadvantages of the known transducers is that they are low in mechanical strength as a whole and especially, the portion which is brought to direct contact with an object being examined is low in mechanical strength. Although the ultrasonic transducer having the construction shown in FIG. 1(a) is improved in mechanical strength over those transducers of FIGS. 1(b) and 1(c), it has the drawback that its sensitivity lowers by 4 to 10 dB.
In certain transducers having constructions similar to those shown in FIGS. 1(a) through 1(c), a protective rubber or resin film is further provided on the side of the acoustic lens 8 which is directly contacted with an object being examined, or between the acoustic lens 8 and the acoustic impedance matcher 7. However, the rubber or resin materials are not favorable from the standpoint of acoustic characteristics: an acoustic impedance thereof is not suitable, acoustic waves attenuate considerably, and/or sensitivity and ring down characteristic lower considerably.
On the other hand, there is known a mechanical scanner-type ultrasonic transducer assembly which comprises an ultrasonic transducer of the construction of FIG. 1(a) or 1(b) encased in a container having an acoustic window. In the container is filled a nearby fluid such as degassed water. In operation, the ultrasonic transducer is mechanically swung so that an object being examined is sector scanned. In this case, the acoustic window which is directly contacted with the object is one of the most important parts of the assembly. The acoustic window must have an acoustic impedance similar to or near the acoustic impedance of the human body (i.e. 1.5 to 1.7.times.10.sup.5 g/cm.sup.2 S) and a reduced degree of acoustic wave attenuation with high mechanical strength. This window is usually made of polyethylene which has an acoustic impedance of 2.3.times.10.sup.5 g/cm.sup.2 S and an acoustic wave attenuation as large as about 1 dB/mm/MHz. The mechanical hardness is as low as about 90 as expressed by Shore hardness A. Thus, the acoustic characteristics and mechanical reliability are not necessarily satisfactory.